No quite spacing out, but that is considered a symptom, but more so of you think of one thing and then because you thought of that you think of another thing and then it goes on and on because you can't get your brain to focus.

i literally lie awake all night thinking about going to the dentist and when i finally fall asleep (I only sleep like 2 hrs a night) i remember it untill i walk out the door then it's like... "oh, i like that fresh sea-breeze in the morning"